The invention relates to a temperature sensor, which is filled with expansion fluid, is connected via a capillary tube, with an expansion member of a regulator, and is adapted for mounting in a fluid container; particularly in a container wall of, for example, a deep fat fryer. Hydraulic sensors are generally better suited for measuring fluid or liquid temperatures, particularly those above 100.degree. C., than electrical or electronic temperature sensors, due to the lower technical expenditure. It is known, for example, to use a straight sensing or sensor tube for measuring fluid temperature, which can also be encased and said tube is allowed to project directly into the liquid. In another known arrangement, the sensing tube is replaced by a box or cell which projects into the liquid. In both cases, it is a disadvantage of the known sensors that their reaction or response time (thermal inertia) is relatively long with respect to the temperature ranges of the medium to be measured. Particularly where powerful heat sources are used for heating the medium, serious difficulties can be caused by this inertia of, for example, 1 to 2 minutes, e.g. the overheating of fat in the deep fat fryer, which is the equivalent of making the medium to be heated unusable. It is therefore necessary in the known constructions to keep the heating capacity correspondingly low, which leads to increased inertia (slower response) of the complete controlled system, which is uneconomical.